Satellite communication and fixed wireless access systems (FWA) in which wireless transmission is performed in a microwave band of 10 GHz or above or in a millimeter wave band mostly employ planar antennas consisting of an array of a large number of patch antennas. A feed line for these patch antennas has a simple structure, so that a parallel feed can be precisely realized at low cost. Moreover, the feed line is formed as a triplate line capable of ensuring high gain and high efficiency.
As shown in FIG. 6, a conventional triplate feed type planar antenna includes, for example, a ground plane 41, a foam sheet 42-1, a flexible substrate 43, a foam sheet 42-2, and a slot plate 44.
The elements stacked in this way are configured as follows.
(1) On an upper surface of the ground plane 41, a pattern corresponding to the ground is formed.
(2) The foam sheets 42-1 and 42-2 are configured as a cushion material, a heat insulating material, and a dielectric with the flexible substrate 43 interposed between both surfaces thereof.
(3) On the flexible substrate 43, an array of rectangular patch antennas 43A1,1 to 43Am,n(m and n are integers) arranged in a grid shape and a feed line 43F which realizes a feed for these patch antennas are formed as a circuit pattern.
(4) Slot openings 44S1,1 to 44Sm,n in a grid shape are formed at positions individually corresponding to the patch antennas in a top surface of the slot plate 44, and a ground pattern is formed all over the surface other than these positions.
A feed system of such a triplate feed type planar antenna is configured, for example, as shown in FIG. 7, as follows.
A waveguide-triplate line converter (simply referred to as “converter” below) 43C is disposed at a predetermined position surrounded by the patch antennas 43A1,1 to 43Am,n on the flexible substrate 43.
Also, on the flexible substrate 43 (FIG. 6), a main line 43B is formed to have one end extending to a probe 43CP inserted from a sidewall of a waveguide 43Cwc constituting the converter 43C into the waveguide 43Cwc and the other end extending to a bus bar 43FM of the aforementioned feed line 43F realizing the feed.
Among the components of the converter 43C, the waveguide 43Cwc (FIG. 7) includes the following elements, as shown in FIG. 8.
(1) A waveguide flange 43CF that corresponds to (i.e. is connected to) one end of a rectangular waveguide connected to a wireless device not shown in the drawings, has a rectangular opening extending into the rectangular waveguide, and is disposed with the opening in contact with the corresponding position of the ground plane 41;
(2) An annular member 43CR that is formed in the stacked foam sheet 42-1, flexible substrate 43, and foam sheet 42-2, inserted into a through-hole corresponding to an imaginary extended portion of the opening, and formed as a conductive cylinder having a through-hole penetrated by the aforementioned probe 43CP (FIG. 7);
(3) An annular member 43Cr that is stacked on the annular member 43CR with the slot plate 44 interposed therebetween, and extends from the inside of the waveguide flange 43CF to the outside of the slot plate 44 together with the annular member 43CR;
(4) A short plate 43Cs that is laid on a top portion of the annular member 43Cr including an opening, and has a top portion in which holes (not shown) penetrated by screws 43 S-1 to 43S-5 to be described below are formed; and
(5) The screws 43S-1 to 43S-5 that sandwich the ground plane 41, the annular member 43CR, the slot plate 44, and the annular member 43Cr between the waveguide flange 43CF and the short plate 43Cs by being inserted into screw holes which are formed in the waveguide flange 43CF to correspond to those holes.
In the ground plane 41, the annular member 43CR, the slot plate 44, and the annular member 43Cr, holes (not shown) into which the screws 43S-1 to 43S-5 are inserted and which have inside walls having a size and a shape with which they stably come in contact with sidewalls of these screws 43 S-1 to 43 S-5 are formed in advance.
In the triplate feed type planar antenna having such a configuration, the waveguide 43Cwc (FIG. 7) is sandwiched by the screws 43S-1 to 43S-5 between the aforementioned waveguide flange 43CF and the short plate 43Cs, and formed by inside walls of the ground plane 41, the annular member 43CR, the slot plate 44, and the annular member 43Cr electrically connected by these screws 43S-1 to 43S-5.
In FIG. 7, the probe 43CP converts transmission waves which are output by a transmitter not shown in the drawings and transferred as an electromagnetic field of the fundamental mode propagating in the waveguide 43CWC into an “electromagnetic field of a triplate line.”
On the flexible substrate 43, this “electromagnetic field of a triplate line” is guided to a point which is deviated from the center of the bus bar 43FM by a distance corresponding to a quarter of a wavelength, and is fed to half of the patch antennas 43A1,1 to 43Am,n and the other half thereof with a phase difference of 180 degrees through the bus bar 43FM.
Also, in such a triplate feed type planar antenna, the phase of the cross-polarization component of one half of the patch antennas 43A1,1 to 43Am,n is opposite to the phase of the cross-polarization component of the other half, and thus the cross-polarization components cancel each other, and thus cross-polarization discrimination is improved.
As prior art relevant to the present invention, there are Patent Literature 1 to Patent Literature 3 listed below.
Japanese Unexamined Patent Application, First Publication No. Hei 09-312515 discloses a polarized wave shared planar antenna “obtained by sequentially stacking a ground conductor, a dielectric, a feed substrate having a plurality of radiation elements and a feed line formed therein, a dielectric, a ground conductor having a plurality of slots installed so that the respective slots are disposed right above the radiation elements, a dielectric, a feed substrate having a plurality of radiation elements and a feed line formed therein, a dielectric, and a ground conductor having a plurality of slots installed so that the respective slots are disposed right above the radiation elements, and configured by electromagnetically coupling the radiation elements and the radiation elements together so that the excitation direction of the radiation elements in accordance with the feed line and the excitation direction of the radiation elements in accordance with the feed line cross at right angles, in which radiation elements corresponding to about half the number of array elements of the feed substrate and the feed substrate and a feed line or elements corresponding to about half the number of array elements of any one feed substrate and a feed line are spatially rotated 180 degrees with respect to a reference excitation direction and disposed, and which electrically changes a feeding phase by 180 degrees to be excited,” thus having a characteristic such that “load on a signal-processing circuit is reduced in a planar antenna side as much as possible, and thus a cross-polarization characteristic and a wide band characteristic of isolation are obtained.”
Japanese Unexamined Patent Application, First Publication No. Hei 11-312909 discloses a waveguide/microstrip line converter “in which a substrate providing a microstrip line so that an antenna probe is disposed toward an opening of a waveguide is sandwiched between a cap corresponding to the opening of the waveguide and a base member, and the microstrip line is connected to both ends of the antenna probe installed on the substrate” thus having a characteristic such that “it also has a function of distributing a feed from the antenna probe in order to reduce removal of antenna elements on the substrate as much as possible.”
Japanese Patent No. 2595339 discloses a planar antenna “that has a triplate configuration in which strip lines formed on a substrate are inserted into a ground substrate with gaps left on both sides, has radiation elements formed on one side of the ground substrate, and supplies power to the respective radiation elements in parallel by a feed line of the strip lines, in which strip line-waveguide converters having strip lines of a final feed point inserted from both side surfaces of a waveguide and having a phase difference of 180 degrees between powers input from both strip lines to the waveguide are formed in spaces between radiation elements horizontally and vertically formed at regular spatial intervals,” thus having a characteristic such that “it enables a feed by a waveguide capable of achieving favorable power combining (branching).”